This invention relates to a radio communication network between a plurality of telephone lines and a plurality of wireless telephone sets or units and, in particular, to a base station for connection to a plurality of substations, such as subscribers, through the telephone lines in the radio communication network. It is to be noted throughout the instant specification that the radio communication network may be a mobile telephone network comprising a plurality of mobile telephone sets, although description will be made only about a cordless telephone network comprising a plurality of cordless telephone sets.
A conventional cordless telephone network generally comprises a base station which is coupled to a single cordless telephone set on communication through a single radio channel preassigned to the cordless telephone set. Thus, the base station is in one-to-one correspondence to the cordless telephone set and is connected to subscribers through a telephone line accommodated in the base station.
Under the circumstances, it might be preferable that consideration is made about installation of a plurality of cordless telephone sets at an individual subscriber in order to respond to demand in market. In this event, a plurality of base stations might have to be installed at the individual subscriber along with a plurality of telephone lines accommodated in th respective base stations and with a plurality of radio channels or waves having different frequencies. The radio waves are used for speech and will therefore be referred to as speech radio waves hereinunder. From the above, it is apparent in the above-exemplified cordless telephone network that each number of the base stations and the telephone lines might have to be equal to the number of the cordless telephone sets.
In addition, the cordless telephone sets must be operable independently of one another and might be simultaneously put into an active state through different ones of the speech radio waves. Preferably, each of the cordless telephone sets may be communicable through every one of the radio waves. Therefore, each of the speech radio waves is probably used in common to the respective cordless telephone sets. Taking this into consideration, a control radio wave or channel must be included in the radio waves to control allocation of the plurality of cordless telephone sets to the speech radio waves.
With such a network, an interference may take place between the control radio wave and a combination of active one or ones of the speech radio waves due to mutual modulation or intermodulation therebetween. Such an interference seriously and adversely affects communication and might ultimately make both transmission and reception operation impossible. It may be considered that similar interference also occurs among the active speech radio channels.
More particularly, let two adjacent ones of the speech radio waves be represented by f.sub.1 and f.sub.2 and be being used now as desired radio waves in the network in question. The radio waves f.sub.1 and f.sub.2 are inevitably supplied to a radio transmitter and a radio receiver which are installed in each of the base stations and the cordless telephone sets and which may collectively be called a radio unit. Inasmuch as such a radio unit usually comprises nonlinear elements, such as transistors, diodes, and the like, having nonlinear characteristics, the desired radio waves are subjected to an influence of nonlinear distortion by the nonlinear elements. As a result, the radio unit produces, in addition to the desired radio waves, undesired frequency components resulting from the nonlinear characteristics of the nonlinear elements. In this case, a serious problem arises from third-order one of the nonlinear distortion that includes intermodulation distortion specified by intermodulation components (2f.sub.1 -f.sub.2) and (2f.sub.2 -f.sub.1), besides harmonic distortion. Overlap of such intermodulation components with the control radio wave makes connection control impossible because the control radio wave is put into a state like an exclusive and objectionable occupation by the intermodulation components. Specifically, there is a strong probability of the above-mentioned overlap when the control radio wave and the speech radio waves are arranged at an equal interval of frequency.
As a rule, faults, such as interference, intermodulation, and the like are liable to occur when a plurality of radio units are adjacent to one another, as well known in the art. In order to avoid such faults, conventional countermeasures are mainly directed to realization of complete shield between the radio units and to use of amplifying elements having excellent linearity. However, such countermeasures result in an increase of costs and an increase of size of each radio unit.